1. Field of the Invention
The invention relates to a wobble plate drive for a cluster of axially reciprocating pistons spaced about a shaft axis; and more particularly to such a drive in which the wobble plate tilt angle with respect to the shaft axis may be varied in order to vary the piston displacement. Drives of this type are known for hydraulic pumps and motors, and have been proposed for multi-cylinder thermodynamic engines.
Because there is not complete uniformity of terminology in this art, the following definitions are applicable to this specification and appended claims.
Swashplate drive refers specifically to a drive having the bearings for reciprocating the individual pistons in direct contact with a swashplate surface that rotates with the shaft. In such a drive, the connection between the piston rod or linkage and the swashplate has both a sliding or rolling relative movement with respect to the periphery of the swashplate, and an oscillating pivoted movement with respect to the swashplate. The sliding or rolling movement is along a surface having circular symmetry about an axis. When the swashplate is tilted so this axis intersects the shaft axis at a given non-zero angle, the oscillating pivotal movement is about a line which normally lies in a plane perpendicular to the swashplate axis. This plane is referred to herein as the driving plane.
A wobble plate drive is a drive having a plate or functionally equivalent member which is coupled through bearings to the bearing surface of a tilted or tiltable element equivalent to a swashplate surface; because of its functional equivalence in the practice of the instant invention, this element will be also referred to as a swashplate. Thus, the word swashplate refers to either a true swashplate or a wobbling element, rotating with a shaft, which is connected by bearings to and defines the wobbling motion of a wobble plate. The wobble plate is restrained from rotation about the shaft axis, and is connected to the piston rods through oscillating pivotal bearings whose pivot axis define a driving plane.
2. Description of the Prior Art
Swashplate drives for hydraulic machines have been in substantial commercial use for a number of years. Because of the high torques and power levels obtainable with hydraulic machines of relatively modest size, extremely rugged shafts and swashplate or wobble plate assemblies have been required. These in turn, are mounted in heavy housings or cylinder assemblies. As a result, only very small vibration amplitudes are transmitted to the machine mounting points.
Many known machines are completely unbalanced insofar as inertia forces caused by piston and piston rod movement and drive mechanism are concerned. For example, it has been common to use a hemispheric thrust plate as the wobble plate or swashplate, the hemispheric assembly itself being relatively massive. An example of this is given in U.S. Pat. No. 3,175,363 which teaches static balancing of an oblique disc used as a wobble plate, and dynamic balancing for overcoming the effect of sudden reduction in the hydraulic load. Dynamic balancing for the torque about an axis perpendicular to and rotating with the shaft due to the inertia of the pistons moving back and forth is not provided. Also, when the plane is tilted, a greater portion of the hemispheric mass is to one side of the shaft than the other, directly producing a radial unbalance.
The significance of radial unbalance has been appreciated and compensated for. For example, in U.S. Pat. No. 3,292,554, a mechanism is disclosed in which a counterweight moves radially to one side away from the shaft as the hemisphere is moved toward the other side of the shaft. Again, however, there is no compensation for the unbalance produced by the inertia forces of the pistons being accelerated in one axial direction at the one side, at the same instant that pistons to the other side of the shaft are being accelerated in the opposite axial direction.
An example of a multi-cylinder thermodynamic engine using a wobble plate drive is given in U.S. Pat. No. 3,319,874. In this patent a relatively complex mechanism is taught which permits varying the stroke of the pistons, while at the same time controlling the compression ratio somewhat independently of the stroke. Because of the lower power and torques encountered in comparison with high pressure hydraulics, the mass of these reciprocating members produces substantial vibration which is less absorbed by a heavy, solid frame. There is, however, no mention or solution of the balancing problem which occurs by reason of the inertia of the pistons and piston rods causing a rotating couple to be produced. An additional embodiment shown in that patent incorporates a hydraulic pump with cylinders directly in line with those of the internal combustion engine, this change clearly being one which will accentuate the problem of vibration due to axial acceleration and deceleration of the pistons.
Similarly, the multi-cylinder hot-gas axial piston engine shown in U.S. Pat. No. 3,538,706 utilizes relatively massive pistons, piston rods and axially moving bearing assemblies for the pistons, so that large uncompensated rotating couples would be produced.
Still more recently, Stirling cycle engines for automotive use have been delivered to Ford Motor Company by N. V. Philips' Gloeilampenfabrieken, incorporating a swashplate drive. These engines have a fixed driving plane tilt angle, and are balanced dynamically by choice of the swashplate mass and dimensions. However, because of the use of a plain swashplate, bearing design is more complex and may be less easily maintained by average automotive mechanics than drives having standard bearing types.